Various mechanisms exist for managing memory in a virtual machine environment. A virtual machine platform typically executes an underlying layer of software called a virtual machine monitor (VMM) which hosts one to many operating system (OS) sessions, also known as virtual mactines (VMs). In existing systems, a VMM controls resources on the virtualization platform, including memory. The VMM takes the amount of physical memory on the platform, i.e. 256 MB or other total, and apportions it (divides it up) among the many. OSs that are hosted in the various VMs. For instance, if the virtual machine session supports Linux and Microsoft® Windows® running simultaneously, and has a total of 256 MB physical available memory, the VMM divides the available total in half for each session. In this example, both Windows® and Linux will each have 128 MB available. Other proportions may be predetermined for the platform. Thus, a priori decision will be made on memory allocation and resource reservation.
A problem with this method is the inability to dynamically create and delete VM sessions efficiently, regarding the use of available physical memory resources. In one example, two VM sessions grows to eight VM sessions and then back to two. In this scenario, the memory must be divided up for the maximum number of sessions, i.e., eight. Thus, when only two sessions are running, ¾ of the available memory is idle and unused. The allocation cannot be changed dynamically, or on the fly.
An operating system (OS) may distinguish between user mode and kernel mode. The OS also has a notion of privileged pages and pageable (swappable) vs. non-pageable (non-swappable) memory. In normal operation of existing systems, some memory is not available for swapping. This is often due to device drivers using specific memory for buffer space. In a virtual environment, the guest OS runs in a VM with knowledge of memory that has been allocated to the guest OS. The guest OS also has knowledge of buffer space required for various device drivers. In a virtual environment of an existing system, virtualization of VM memory is not possible by a virtual machine monitor because some pages are non-swappable. In existing systems, only the guest OS knows which pages are non-swappable and thus, only the guest OS may perform virtual memory operations. The VMMs of existing systems will not be able to accommodate memory that is not available for swapping, with respect to the guest VM OS. The VMM of existing systems allocates physical memory to the guest OS and the OS keeps track of non-swappable areas.